Field of the Invention
The present disclosure relates to an image heating apparatus included in an image forming apparatus such as a copying machine and a printer, and, in particular, to an apparatus configured to heat an image by electromagnetic induction heating with use of a high frequency.
Description of the Related Art
Conventionally, there is provided an image forming apparatus such as a copying machine and a printer of the electrophotographic method or the like that includes an image heating apparatus configured to heat and fix an unfixed image (a toner image) formed on a recording material such as printing paper and an overhead projector (OHP) sheet by an appropriate image formation process, onto a surface of the recording material as a permanently fixed image. One of methods employed for the image heating apparatus is the electromagnetic induction heating method. This type of image heating apparatus includes a heated member configured to generate heat by an induced current and an exciting coil configured to produce a magnetic flux, and heats the unfixed image on the recording material with the aid of the heat of the heated member. As such a fixing apparatus, there is discussed a fixing apparatus in which a part of a core configured to form a closed magnetic path is inserted through a hollow portion of a roller-like heated member, and an alternating current of a low frequency (50 to 60 Hz) is supplied to an exciting coil helically wound around the core so that the roller-like heated member is heated (see Japanese Patent Application Laid-Open No. 10-319748).
Generally, a transformer can be downsized by an increase in a driving frequency with use of a switching power source or the like. The reason therefor is that the increase in the driving frequency can reduce a magnetic flux required to produce a same voltage, thereby allowing a magnetic core to be designed so as to have a small cross-sectional area.
However, in the fixing apparatus discussed in Japanese Patent Application Laid-Open No. 10-319748, the increase in the driving frequency raises the following problem. Relatively high power of several hundred watts or higher should be produced in the image heating apparatus included in the image forming apparatus. Therefore, the exciting coil has a large number of turns, and a parasitic capacitance (also referred to as a stray capacitance or a floating capacitance) tends to be formed between adjacent coil wires. This parasitic capacitance behaves as if a capacitor is connected in parallel with the exciting coil. As a result, if an alternating current of a high frequency (a frequency range from 20.5 kHz to 100 kHz) is supplied to the exciting coil with use of a switching power source using a resonance circuit, a switching loss and a switching noise may increase according to an undesired charge to and discharge from the parasitic capacitance, resulting in breakage of the power source.